Uncoupled Auxiliary S2-Alar-Iliac Screws in Long Lumbopelvic Constructs: Exploratory Finite Element Analysis and Preliminary Clinical Experience

Background and Objectives: In this study, uncoupled auxiliary trans-sacroiliac (UATS) screws were evaluated as a mechanically independent adjunct to conventional S2-alar-iliac (S2AI) fixation in long lumbopelvic constructs. We assessed their exploratory biomechanical behavior and preliminary clinical feasibility. Methods: Conventional bilateral S2AI fixation and the same construct supplemented with bilateral UATS screws were compared in an exploratory finite element analysis under flexion, extension, lateral bending, and axial rotation. A retrospective clinical series of 26 patients undergoing UATS-supported lumbopelvic reconstruction was also evaluated. Results: Across the four loading modes, the UATS configuration showed lower global implant-system stress (81–92% reduction) and lower global sacral stress (43–67% reduction) within the computational model. The clinical cohort was revision dominant (25/26 patients). Mean SIJ-related pain VAS decreased from 8.0 ± 1.5 to 2.4 ± 1.0 and ODI from 68.1% ± 18.1% to 16.7% ± 8.6% at final follow-up (both p < 0.001). These clinical changes reflect the overall reconstructive procedure and cannot be attributed independently to UATS fixation. Conclusions: UATS placement was technically feasible in this small, revision-dominant cohort and altered stress distribution in an exploratory, unvalidated finite element model. The present study does not establish independent clinical efficacy or superiority over established pelvic fixation or SIJ fusion strategies.